1. Field of the Invention
This invention relates to apparatus for testing cathode ray video display tubes (CRTs), and particularly to apparatus for generating a video pulse signal which produces on the video screen of a CRT a scanning test line displaying the spot size and line focus of the electron scanning beam of such CRT. Line focus can thereby be adjusted and measured at any desired light output, using the same video signal.
2. Description of the Related Art
A problem encountered in mass production of CRT display tubes for television receivers and video monitor terminals is determination of whether the beam is correctly focussed on the display screen. Conventional practice is to assume that if the beam current conforms to established specifications the beam will be in correct focus. However, due to variations in CRT design and construction or, in the case of a color CRT, the positioning of the shadow mask, there may be considerable variation in spot size, and consequently in scanning line focus, on the face of the display screen even with a given value of beam current.
In order to adjust line focus, the procedure has heretofore been to apply a video signal to the CRT which produces a specific test pattern, such as an Indian head, on the video screen. The operator then adjusts the focusing components of the CRT and the current supplied thereto until the lines in the test pattern appear as sharp as possible. The accuracy of this procedure obviously depends to a large extent on the skill of the operator, resulting in considerable variation in line focus and spot size from one CRT to another. At maximum beam current, corresponding to maximum brightness of the beam spot on the video screen, the size of the spot increases or "blooms", and if excessive, will result in loss of image detail.